As one can ascertain, filters are widely employed to provide a desired specified response to a given input signal. Most filters are used to allow certain frequencies to be transmitted to the output load whine rejecting the remaining frequencies. Many designs demand not only a specified amplitude response, but a specified phase response as well. Filters can be designed in either the frequency domain or the time domain, although the former approach is the more fully developed. The design of microwave filters owes much to the progress made in the design of lumped parameter filters. Thus there are many texts that refer to design techniques for various types of microwave filters.
Almost all electronic systems use filters including systems at microwave frequencies. Microwave filters basically differ in construction from lumped filters which operate at lower frequencies because wire inductors have high losses (i.e. low Q factors) at microwave frequencies and, therefore, are impractical. Typically low-loss high-performance microwave filters use quarter-wavelength long structures such as waveguide cavities, transmissions lines or hybrids of both to form resonators in lieu of employing inductors and capacitors. These resonators when coupled together provide a frequency dependent response or filter response. Filters made employing prior art techniques can be quite large, for example, at 3 GHz a quarter-wavelength is 2.5 cm. long.
One of the system applications for tunable microwave filters is in Electronic Warfare (EW) systems. One of the problems in EW systems is neutralizing jamming signals. All that is required to jam a system is to send a frequency signal somewhere in the system's operational band. Typically, preselector filters at the system's front end eliminate unwanted signals from out of band and a tunable notch filter can be added to eliminate any unwanted signals which are in the band. However, adding an extra filter at the input of the system increases insertion loss which in turn decreases system performance. The extra filter also increases the system's weight and size which could be substantial based on the above-noted characteristics. This essentially is undesirable for many applications such as for systems used on aircrafts and so on. Finally, isolation is required between the two filters to insure full performance from each. The problem is magnified by one of the filters being tunable. If the notch filter operates at a fixed frequency, then the filters can be tuned to operate as a pair. In any event, since the notch changes frequencies the filter characteristics change.
Isolation can be provided at microwave frequencies by a ferrite device call an isolator. These devices require magnets and are usually relatively heavy. Therefore, the combination of a tunable notch and a bandpass filter separated by an isolator exceeds the weight and size requirements of many microwave EW systems.
It is therefore an object of the present invention to provide an improved microwave bandpass filter including an integral tunable notch filter characteristic, which avoids many of the prior art problems.